In an ejector, a primary high-velocity fluid, such as a steam, gas, or vapor jet, is used to entrain and pump a secondary low-velocity fluid while mixing with it. In an ejector, the mixing of the primary high-velocity fluid and the secondary fluid occurs in the mixing section of the diffuser by sheer forces between the high-velocity stream and the secondary low-velocity fluid. Ejectors have a low power conversion efficiency due to the dissipation of energy resulting from friction forces between the primary high-velocity fluid stream and the secondary low-velocity fluid.
Jet engines create thrust by directing a high-energy exhaust stream from an exhaust nozzle. Typically, a jet engine accepts air through an inlet and compresses the air in a compressor section. The compressed air is directed to a combustion chamber, mixed with fuel, and burned. Energy released from the burning fuel creates a high temperature in the combustion chamber. The high-pressure air passes through a turbine section and into an exhaust chamber. The high-pressure air is then forced from the exhaust chamber through a nozzle, where the air exits the engine. Typically, as the air passes through the throat of the nozzle, it expands and accelerates from subsonic to supersonic speeds, essentially translating the energy of the exhaust flow from a pressure into a velocity. The energy level of the air in the exhaust chamber generally relates to the velocity of the air as it exits the nozzle. The greater the velocities of a given mass flow of air exiting the engine, the greater the thrust created by the engine.
High performance aircraft commonly augment the energy level of the air in the exhaust chamber by using an after-burner. After-burners add fuel to the exhaust chamber and ignite the fuel in the exhaust chamber. This increases the temperature of the exhaust flow. Although the energy added by after-burn fuel can greatly increase the thrust of the engine, the reduced density of the hotter air requires a larger nozzle effective throat area. Failure to increase the nozzle effective area during after-burning with a typical jet engine can cause excessive backpressure in the compressor section and turbine section, causing the engine to stall. To alleviate these difficulties, jet engines with after-burners typically use variable geometry nozzles to throttle the exhaust flow from the exhaust chamber. When an after-burn is initiated, the circumference of the nozzle's throat is increased to increase the cross-sectional flow area through the throat. This increased cross-sectional maintains a reasonable pressure in the exhaust while accommodating higher temperatures. Modern after-burning jet engines with variable geometry nozzles can require as much as a two-fold increase in cross-sectional throat area to maintain constant engine flow and back-pressure in response to the extra thermal energy added by the after-burner.
Although variable geometry nozzles allow the use of an after-burner, they also have many inherent disadvantages, which penalize aircraft performance. For instance, a variable geometry nozzle can be a significant component of the weight of an engine. Such nozzles are typically made of large, heavy metal flaps, which mechanically alter nozzle geometry by diverting exhaust flow with a physical blockage and thus have to endure the high temperatures and pressures associated with exhaust gases. In an IRIS type nozzle, typically used on after-burner equipped engines, the actuators used to adjust the nozzle flaps to appropriate positions in the exhaust flow tend to be heavy, expensive and complex because of the forces presented by the exhaust flow which the nozzle must overcome.
Further, the nozzle flaps typically constrict the exhaust flow by closing and overlapping each other, which allows hot air to escape between the flaps. These leaks cause reduction in thrust. Additionally, variable geometry nozzles are also difficult to implement on exotic nozzle aperture shapes typical of an advanced tactical fighter aircraft.
One method of overcoming this weight restriction is the use of a fixed geometry nozzle in a jet engine to inject a secondary flow of high-pressure air across the primary flow as the primary flow passes through this nozzle. The secondary flow can partially block the exhaust exiting the nozzle to increase the pressure within the exhaust chamber. When an over-pressure exists in the exhaust chamber, the secondary flow can be reduced to increase nozzle throat area and reduce the nozzle pressure.
Although the injection of a secondary flow will support a fixed geometry nozzle in an after-burning jet engine, this method also introduces inefficiencies to the operation of the engine. Primarily, the injection of air across the flow of the exhaust tends to use a large amount of high-pressure air to obtain an effective nozzle blockage. Thus, injection can introduce inefficiencies as the total momentum of the exhaust flow is decreased by the decreased flow from the compressor section into the combustion section if compressed air is bled from the compressor section for injection. This inefficiency can result in a reduced range of operations for a given fuel supply and fuel flow.